When a plurality of packet senders are present in an identical packet transmission line as in cable or wireless LAN, the send of packets in the same timing sometimes causes collision of the packets against each other. The collision of data (packets) against each other results in breaking of send data, or otherwise makes it impossible to guarantee the quality of send data. In order to overcome this problem, for example, the following method has hitherto been used to avoid the collision of data.
CSMA/CD (carrier sense multiple access with collision detections is known as a method for detecting the collision of data and controlling and managing data. According to this method, prior to transmission of data, CS (carrier sense) is performed to detect carriers on the transmission line, and, when, upon the send of data, the data has collided against other data on the transmission line, CD (collision detection) is performed to detect a collision signal. Upon the detection of the collision signal, terminal units, which have sent the collided data, resend the respective data after standby, i.e., after the elapse of random times generated by the respective terminal units. According to this CSMA/CD method, the random time is determined by a calculation method, which can logarithmically reduce the probability of collision, so as not to cause collision again.
Another method is ICMA/PE (idle-signal casting multiple access with partial echo). The idle/occupy of a leading channel has been indicated by idle line/inhibit bit, and the receive/non-receive due to collision has been indicated by collision detection indication bit. By contrast, according to this method, in addition, send data subjected to certain processing is folded back as a partial echo to the trailing collision control bit, and, thus, sending can be confirmed frame by frame.
For example, Japanese Patent Laid-Open No. 128059/1993 discloses a priority data transfer method wherein the backoff time is determined in such a manner that, based on whether collided packets are usual data or priority data, priority data is judged, and the packet judged to be priority data is preferentially resent.
FIG. 13 is a schematic block diagram showing a conventional priority data transfer method.
According to the conventional priority data transfer method shown in FIG. 13, a communication unit 1310 is connected to a communication unit 1320 through a bus 1330. The communication unit 1310 comprises a collision detector 1311, a resend time generation section 1312, a priority data judging section 1313, and a random number generation section 1314. The communication unit 2 has the same construction as the communication unit 3.
In this conventional priority data transfer method, as soon as the collision detector 1311 has detected that collision on the bus 1330 continuously occurred a predetermined number of times, the collision detector 1311 requests the resend time generation section 1312 to generate a send data resend time of the data, which could not have been sent due to the collision on the bus 1330, and, in addition, requests the priority data judging section 1313 to judge the priority of the send date, which could not have been sent due to the collision on the bus 1330.
The resend time generation section 1312 requests the random number generation section 1314 to generate a random number, and generates a resend time of the send data corresponding to the random number generated in the random number generation section 1314.
The priority data judging section 1313 checks the priority of the send data requested by the collision detector 1311 to judge whether the requested send data is ordinary data or priority data, and informs the resend time generation section 1312 of the result.
In the conventional priority data transfer method, however, judgment processing in the priority data judging section for judging whether or not the collided data is priority data is not embodied.
Further, the conventional priority data transfer method is used for avoiding the collision of data between communication units connected to each other through a bus (a cable), and no mention is made to radio communication.